1. Field of the Invention
This invention relates to a control device used in electrophotographic image forming devices, for controlling a high voltage power source which generates a high voltage required for forming an electrophotographic image, for example, and more particularly to a control device for controlling a high voltage power source which generates the high voltage by switching a primary current in a transformer provided therein.
2. Description of the Related Art
A copy machine and laser printer are known examples of an electrophotographic image forming device. An image forming device typically includes a photosensitive drum serving as an image carrier, for example, and it additionally includes a charging section, an exposing section, a developing section, and a transferring section arranged to surround the photosensitive drum. In the image forming process, the photosensitive drum is rotated in one direction and sequentially subjected to corresponding processes in the charging section, exposing section, developing section, and transferring section. The charging section uniformly charges the surface of the photosensitive drum, the exposing section selectively exposes the drum surface to create an electrostatic latent image, and the developing section charges toners and supplies the charged toners to the drum surface so as to develop the thus created electrostatic latent image. The toners are attached to the electrostatic latent image on the surface of the photosensitive drum to convert the electrostatic latent image into a visible toner image. The transferring section charges paper and supplies the same to a position at which image transfer is effected, thereby to transfer the image formed on the drum surface to the paper. The image forming device further includes a plurality of sets of high voltage power sources and high-voltage power source control devices for stably supplying high voltages of approx. 5 kV which are required in the electrostatic charger, developing sleeve and transfer charger respectively arranged on the charging section, developing section, and transferring section. As shown in FIG. 1, each of the high voltage power sources includes a high voltage generating transformer for generating a high voltage and a switching circuit for periodically switching the primary current of the transformer.
The conventional high-voltage power source control device has the following construction for controlling the high voltage power source. The control device includes a switching controller 10, an A/D converter 12, and a monitor circuit 14 which are connected to one another as shown in FIG. 1. The switching controller 10 includes a counter for counting clocks and generates a switching pulse each time the output of the counter reaches a value indicating a preset switching period suitable for the high voltage power source HV. In the high voltage power source HV, the switching circuit permits a primary current to flow in the high voltage generating transformer for the duration of a switching pulse supplied from the switching controller 10 and the high voltage generating transformer generates a high voltage according to variation in the primary current. An output voltage of the high voltage generating transformer, i.e., an output voltage of the high voltage power source, rises or falls according to variation in the load, and the rising or falling thereof being detected by a monitor circuit 14. An output voltage of the monitor circuit 14 is supplied to the A/D converter 12 as a monitor voltage. The A/D converter 12 repeatedly converts the monitor voltage into a digital signal which it supplies to the switching controller 10 as monitor voltage data. The switching controller 10 holds reference voltage data representing a reference value of the output voltage of the high-voltage power source and adjusts the duty ratio of the switching pulse according to a difference between the reference voltage data and the monitor voltage data. This adjusting operation is repeated each time the monitor voltage is subjected to the A/D converting process so as to hold the output voltage of the high voltage power source near the reference value. In the process of adjusting the duty ratio, the difference between the reference voltage data and the monitor voltage data is added to the duration data of the switching pulse and the result of addition is used to specify the duration for succeeding switching pulses. If the result of addition exceeds one half the switching period, the duration of the switching pulse is set to one half the switching period and thus the maximum value of the duty ratio is limited to 50%.
In order to stabilize an output voltage of the high voltage power source by the high-voltage power source control device, it is necessary to set the relation that the 1-LSB resolution of the A/D converter is lower than the 1-LSB resolution of the counter. When the high voltage power source is used to supply a high voltage of 5 kV to the load of the electrostatic charger, developing sleeve, transferring charger, or the like, a voltage variation of .+-.50 V is permitted. If the precision of .+-.1/2 LSB of the A/D converter is set to fall within the permissible range of the voltage variation, the 1-LSB resolution of the A/D converter becomes 100 V for an output voltage of the high voltage power source. In this case, the reference value (=5 kV) is represented by 5 kV/100 V=50 (i.e., 32h, where h is a hexadecimal number). In order to represent the reference value, 6 bits are necessary, but normally, a general type 8-bit A/D converter, which is not so expensive as a 6-bit A/D converter is used as the A/D converter. In order to set the maximum duty ratio of the switching pulse to 50% and obtain 5 kV, a switching period of 25 .mu.s is necessary. The duration of the switching pulse must be changed by 50 ns in order to change the output voltage of the high voltage power source by 100 V by a single adjustment of the duty ratio. If the 1-LSB resolution of the counter corresponds to 50 ns, the switching period (=25 .mu.s) can be expressed by 25 .mu.s/50 ns=500 (i.e., 1F4h, where h is a hexadecimal number). Therefore, the counter must be of 9-bit type in order to represent the above value. If the 1-LSB resolution of the counter is set to be higher than the 1-LSB resolution of the A/D converter, variation in the output voltage of the high voltage power source can be stabilized near the reference voltage by the adjustment of the duty ratio of the switching pulse, as shown in FIG. 2.
However, in this case, the high-voltage power source control device has the following defects arising from the relation that the resolution of the counter is higher than that of the A/D converter. Specifically, when plural units of high-voltage power source control devices are manufactured, variation in the control ability thereof due to variation in the performance of the A/D converters used becomes relatively large, and this undesirable from the viewpoint of device reliability. One measure considered in order to overcome the above drawback was to reduce an error in the conversion of the monitor voltage by setting the 1-LSB resolution of the A/D converter approximately equal to the 1-LSB resolution of the counter, for example. However, use of this measure created a new problem in that the output voltage of the high voltage power source could not easily be set stable in relation to the reference voltage as shown in FIG. 3. The problem could be overcome by increasing the number of bits of the counter, together with the clock frequency, so as to set up the desired relation. However, unlike the A/D converter, the cost of the counter and the peripheral circuit thereof became significantly higher due to the increase in the bit number because it is necessary to attain the high speed operation. Therefore, increasing the bit number of the counter is not an effective solution, since it drives up the manufacturing cost.
Variation in the control ability caused by a difference in the performance of A/D converters used will be explained. FIG. 4 shows the input/output characteristic of three A/D converters. Each of the three A/D converters is designed to convert a monitor voltage to a digital value in units of 0.1 V and has a precision or conversion error of .+-.0.05 V (=1/2 LSB). When the monitor voltage of 2.5 V is set for the reference voltage (=5 kV) of the output voltage of the high voltage power source, the first A/D converter converts a monitor voltage of 2.45 V to 2.55 V to monitor voltage data of "80h", for example, as shown by a solid line, the second A/D converter converts a monitor voltage of 2.40 V to 2.50 V to monitor voltage data of "80h", as shown by broken lines, and the third A/D converter converts a monitor voltage of 2.50 V to 2.60 V to monitor voltage data of "80h" (h is a hexadecimal number), as shown by one-dot-dash lines. In other words, the range of the monitor voltages each of which is converted to the same monitor voltage data differs in accordance with the varying process conditions under which individual A/D converters were manufactured. As a result, an output voltage of the high voltage power source is set stable in a range of (5.0 kV.+-.50 V), (5.05 kV.+-.50 V), or (4.95 kV.+-.50 V) in the case of A/D converters with a conversion error of (2.5 V.+-.0.05 V), (2.55 V .+-. 0.05 V), or (2.45 V.+-.0.05 V), respectively, for example. In the case of the high-voltage power source control devices made up of plural units, the central value of the stabilized range, i.e., the target value varies in a range of (the reference value.+-.50 V), as shown in FIG. 5, even when the conversion error of the A/D converters used is uniform.